Are Anion Radicals Nucleophiles and/or Outersphere Electron Donors? An Ab Initio Study of the Reaction of Ethylene and Formaldehyde Anion Radicals with Methyl Fluoride and Chloride

Ab initio calculations combined with an intrinsic
reaction coordinate analysis of the title reactions have
allowed the identification as well as the geometric and thermodynamic
characterization of two types of reaction
paths on the same potential energy hypersurface. One involves an
outersphere electron transfer from the anion
radical which dissociatively reduces the methyl halide acceptor into
the methyl radical and the halide ion. The other
pathway leads to substitution (two pathways leading to O- and
C-substitution repectively in the case of
CH2O•-).
The substitution transition states have a tighter geometry than
the outersphere transition state and may be viewed
equivalently as SN2 reactions or innersphere electron
transfers concerted with bond cleavage and bond formation.
In the competition between the two types of processes, because of
bonded interactions in the transition state,
SN2
substitution has a lower energy barrier than the outersphere reaction.
Owing to the looser geometry of its transition
state, entropy favors the outersphere pathway. This factor is not
sufficient to reverse the free energy balance with
the reactants investigated but this is predicted to occur with
sterically hindered systems.